Bidrawn, Fred
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Publication Investigation and Improvement of SofC Composite Cathodes(2010-12-22) Bidrawn, FredThe focus of this dissertation is on the preparation, performance, and long term stability of SOFC composite cathodes prepared by infiltration methods. The majority of the work that follows aims to improve the understanding of the processes contributing to cathode deactivation and to propose strategies to lessen the extent of this deactivation. Through this understanding of the factors governing cathode performance, improvements can be made in overall cathode performance which can in turn lead to lower operating temperatures. The fuel cells used in this work were prepared by tapecasting and infiltration methods. Composite YSZ-perovskite electrodes were prepared by infiltration of stoichiometric ratios of perovskite precursor nitrate salts into a porous YSZ scaffold. First, the influence of ionic conductivity on the performance of solid oxide fuel cell cathodes was studied for electrodes prepared by infiltration of 40-wt% La0.8Ca0.2FeO3 (LCF), La0.8Sr0.2FeO3 (LSF), and La0.8Ba0.2FeO3 (LBF) into porous YSZ scaffolds. Although ionic conductivity varied by over an order of magnitude, no significant difference was observed in the performance of each material, suggesting that oxygen ion diffusion through perovskite film is not a rate limiting step for the oxygen reduction process within the cathode. Next, the effect of various infiltrated dopants on the performance of SOFC cathodes was examined. The addition of dopants had little influence on the 1123-K composite electrodes but all dopants tested improved the performance of the 1373-K, suggesting that the improved performance is related to structural changes in the electrode, rather than to improved catalytic properties or ionic conductivity. Based on these results, a model was developed to understand the performance of these electrodes. Two rate-limiting cases are considered for oxygen transfer into the YSZ fins: diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. In agreement with the experimental results, an important implication from the model is that ionic conductivity of the pervoskite phase does not limit performance, for most commonly used perovskites, and that surface adsorption is likely limiting. Finally, strategies for improving cathode performance and stability are discussed.Publication Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration(2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond JA mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.Publication Doped-Ceria Diffusion Barriers Prepared by Infiltration for Solid Oxide Fuel Cells(2010-05-24) Küngas, Rainer; Bidrawn, Fred; Vohs, John M; Gorte, Raymond JTo stabilize solid oxide fuel cells cathodes prepared by infiltration of La0.8Sr0.2CoO3 (LSCo) into porous yttria-stabilized zirconia (YSZ), a coating of Sm-doped ceria (SDC) was first deposited onto the YSZ scaffold. The dense SDC coating was prepared by infiltration with aqueous solutions of SM(NO3)3 and Ce(NO3)3, followed by calcination to 1473 K. The SDC coating prevented ~ 20 mΩ cm2, at 973 K, with acceptable degradation after heating to 1373 K.Publication Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration(2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond JA mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.Publication Fabrication of LSM-YSZ Composite Electrodes by Electrodeposition(2010-01-01) Bidrawn, Fred; Vohs, John M; Gorte, Raymond JComposites of Sr-doped LaMnO3 (LSM) and yttria-stabilized zirconia (YSZ) were prepared by sequential electrodeposition of La and Mn species from nitrate salts in Dimethyl Sulfoxide (DMSO) into porous, carbon-coated YSZ substrate, followed by the infiltration of Sr(NO3)2. The La and Mn species were uniformly deposited throughout the depth of 50 µm porous layer to the interface with a dense YSZ electrolyte. The LSM perovskite phase was formed after heating to 1373 K. Solid oxide fuel cell cathodes prepared by single-step electrodeposition showed similar performance to LSM-YSZ electrodes prepared by wet impregnation using many steps.